# Homework Help: Find distance of focus point (conceptual)

1. Apr 29, 2015

### Shinwasha

1. The problem statement, all variables and given/known data
A lens has a radius of curvature of magnitude R on its left surface and a radius of curvature of magnitude 2R on its right surface. Parallel rays entering the lens from the left are focused to a point 135mm to the right of the right surface.
At what distance from the lens is the point at which parallel rays entering from the right are focused?
2. Relevant equations

f=r/2

3. The attempt at a solution
My first attempt was to find the R value

This gives me
f1= r/2
f2= 2r/2 = r

f1=r/2 = 135*2 = r = 270
f2=2r/2 = 135 = r

I'm lost with conceptual problems. Also last question for the year and just want to say thank you to everyone that's helped. On to Quantum Physics next semester :)

2. Apr 30, 2015

### Simon Bridge

You need to be familiar with the physics ... the reasoning is supposed to go like: R->2R->f so 2R->R-> f/2 or 2f or 4f or something.
If the two surfaces had the same R, then the focal length would be the same in both directions.
What do you think making the far surface less curved does to the focal length?
Is this the same as making the near surface more curved?

You can get a feel even just by sketching ray diagrams.
If you have some fat-lens formulae, you should have a play around with them.
Try expressing f1/f2 in terms of R1/R2

3. Apr 30, 2015

### rude man

Are both surfaces convex?

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